For many UK planetary scientists, the end of the year and the start of the next millennium will be marked not only by the usual seasonal celebrations but also by a unique feast of data from two NASA space missions.

As they swoop past Jupiter, the Galileo and Cassini spacecraft will undertake the first dual investigation of the largest planet in the Solar System, its family of moons and huge magnetic field.

THE JUPITER MILLENNIUM MISSION

Despite the handicap of a main antenna that failed to deploy, Galileo has been sending back a stream of images and other information on the Jovian system during the past five years.

Overcoming severe radiation damage, the redoubtable orbiter has deployed a probe into Jupiter's atmosphere; studied the hottest volcanoes in the Solar System on Jupiter's moon Io; discovered evidence of subsurface oceans on the moons Europa and Ganymede; and provided new insights into the giant planet's stormy atmosphere, gigantic magnetosphere and energetic particle environment.

Meanwhile, the Cassini spacecraft has been heading inexorably away from the Sun and Earth during its seven year odyssey to the beautiful ringed planet Saturn. On 30 December 2000, Cassini will pass within 10 million km of Jupiter in order to pick up speed through a so-called gravitational assist. Cassini will approach on Jupiter's sunlit side, then move away on the dark side of the planet.

While Cassini remains at a respectful distance to avoid the planet's hazardous radiation belts, Galileo will loop around Jupiter on a highly elliptical (egg-shaped) orbit, passing close to Jupiter's largest moon, Ganymede, on 29 December.

Scientists around the world have recognised the unique opportunity to carry out the first joint spacecraft study of Jupiter - dubbed the Jupiter Millennium Mission by NASA. Between early October 2000 and March 2001, instruments on both Cassini and Galileo will be investigating the Jovian system.

The main objectives of the Millennium Mission will be:

To examine the interactions between the electrically charged particles of the solar wind and Jupiter's magnetosphere (the region of space dominated by its powerful magnetic field). While Galileo takes measurements from within the magnetic bubble, Cassini will be flying outside, in the region dominated by the solar wind.

To use Cassini's advanced imaging instruments to study the weather and changing cloud formations in Jupiter's atmosphere.

To observe Io and Ganymede with both spacecraft during eclipses in order to learn more about their tenuous atmospheres.

To study Io's torus - a doughnut-shaped cloud of charged particles that are ejected from Io into orbit around Jupiter.

To use Cassini's spectrometers to study the surfaces of Jupiter's moons.

To map changes in the powerful radio emissions from Jupiter's magnetosphere.

To determine the rotation period of the small moon Himalia.

To study the particles and structure in Jupiter's dark rings.

UK PARTICIPATION

Researchers from a number of UK universities who have played leading roles in one or both of these groundbreaking missions are looking forward to a scientific bonanza over the next few months.

IMAGING OF JUPITER'S ATMOSPHERE AND MOONS

Professor Fred Taylor of Oxford University is a co-investigator on Galileo's NEAR-INFRARED MAPPING SPECTROMETER (NIMS), an instrument designed to study Jupiter's cloud formations and atmospheric processes. Co-investigator on NIMS and Cassini's COMPOSITE INFRARED SPECTROMETER (CIRS) is Oxford's Dr. Simon Calcutt.

NIMS has already provided images and information on the composition and temperature of atmospheric features, such as the Great Red Spot, and "hot spots" in Jupiter's all-enveloping blanket of cloud.

"With NIMS, we have been able to probe Jupiter's cloud structure and map its atmospheric composition," said Professor Taylor. "No less than four different cloud layers make up the visible face of the planet, from water clouds at depth, through frozen ammonia and hydrogen sulphide to hydrocarbon haze on top.

"The CIRS is a much more capable instrument than NIMS, with much higher spectral resolution and greater wavelength coverage," he explained. "One of the things that is still eluding us is the nature of the chemicals that colour the Great Red Spot. We hope the data from Cassini will shed new light on this."

Professor Carl Murray of Queen Mary, University of London, is a member of the IMAGING SCIENCE SUBSYSTEM (ISS) team on Cassini.

"From mid-December, ISS will be taking images on a daily basis," said Professor Murray. "Many images of the planet are being taken at wavelengths not available to Galileo and Voyager."

"There are two other main targets for the ISS cameras: rings and satellites," he added. "A major goal is to observe the Galilean satellites in eclipse. We have a much more distant encounter with the main moons than the very close Galileo flybys, but we will still have a spatial resolution about twice as sharp as Galileo's images during eclipse observations of Io. The idea is to look for Io's active volcanoes and other hot spots.

"We are also searching for new, small moons near the planet and we get the closest any spacecraft has ever been to Himalia, the largest of Jupiter's outer moons.

"A joint study by Cassini and Galileo will help us to determine the three-dimensional structure of Jupiter's rings and will give us a good idea of the particle sizes in the rings. We will also be making ring movies to see how their structure evolves with time."

MAGNETIC FIELD STUDIES

Professor David Southwood of Imperial College, London is the principal scientific investigator for Cassini's DUAL TECHNIQUE MAGNETOMETER (MAG) and a co-investigator on Galileo's magnetometer experiment. ICL's Dr. Michele Dougherty is also a co-investigator on MAG.

The MAG instrument provided some important new results during Cassini's 1997 Earth encounter, while Galileo's magnetometer has returned important evidence of subsurface oceans on Europa and Ganymede.

"We are not certain whether Cassini will enter into the magnetosphere," said Dr. Dougherty. "However, we are very excited about the observations, partly because, at the same time that Cassini is in the solar wind, Galileo will be taking measurements inside the Jovian magnetosphere - the first time two point measurements will have been made at a planet other than the Earth."

PLASMA ENVIRONMENT AND MAGNETOSPHERES

Dr. Andrew Coates of Mullard Space Science Laboratory (part of University College, London) is the team leader for the ELECTRON SPECTROMETER, and a co-investigator on CASSINI'S PLASMA SPECTROMETER (CAPS) instrument.

CAPS measurements during Cassini's fast Earth flyby provided some surprising results that scientists are still trying to explain. It has also observed the solar wind at a range of distances from the Sun.

"There are clear signs in the CAPS-ELS electron, magnetometer and wave data that Cassini saw the effects of the huge solar eruption on 8 November, when it was over four times the Earth's distance from the Sun," said Dr. Coates. "The dramatic effects of a coronal mass ejection ploughing through the interplanetary medium were also seen by Cassini on 18-23 November.

"Such big disturbances in the solar wind may well cause Jupiter's magnetosphere to flap around significantly, leading to fluctuations in its aurora," explained Dr. Coates. "Cassini provides a unique platform to monitor these changes.

"During the Jupiter encounter, the solar wind conditions will dictate whether Cassini crosses the bow shock and enters the magnetosphere. At the very least our measurements, and those of the magnetometer, will provide a good upstream monitor for Galileo, which will be inside the magnetosphere, and for the Hubble Space Telescope, which is looking at the aurora from Earth orbit.

Professor Tony McDonnell of the Open University is a co-investigator on Galileo's DUST DETECTION instrument. He is also one of the investigators on Cassini's COSMIC DUST ANALYSER (CDA), which is designed to measure the physical and chemical make-up of small dust particles. Rutherford Appleton Laboratory in Didcot contributed to the design and manufacture of CDA.

The instrument has already detected dust between the planets, including an unprecedented burst of 70 particles on its way towards Jupiter.

"We've been operating between the planets for 18 months and analysed a handful of particles. Then, on 5 September, we got 70 in one day!" said Professor McDonnell. "They must be particles ejected from Jupiter - as Galileo found earlier - but the amazingly clear spectra from CDA will now enable us to find out what they are made of."

RADIO and PLASMA WAVES

The RADIO and PLASMA WAVE SPECTROMETER instrument on Cassini is used to investigate electric and magnetic field waves generated by the electrified gas (plasma) that flows away from the Sun and interacts with the planet's magnetic field. Dr. Hugo Alleyne from the University of Sheffield is a co-investigator on this instrument.

"The RPWS will be looking for the sources of lightning and other powerful sources of radio noise generated in Jupiter's magnetosphere," said Dr. Alleyne. "These are caused by complex interactions between the solar wind, the magnetosphere and the moons."

TITAN PROBE

Attached to the Cassini mother craft is the European Space Agency's HUYGENS probe, whose task is to parachute onto the unexplored surface of Titan, the largest of Saturn's moons (see RAS press notice 97/36). Principal scientific investigator for the Surface Science Package on Huygens is Dr. John Zarnecki of the Open University.

NOTES:

Galileo was launched from the Space Shuttle on 18 October 1989 and made two swingbys of Earth (8 December 1990 and 8 December 1992) and one of Venus (10 February 1990) on its way to Jupiter. Since entering orbit around Jupiter on 7 December 1995, it has survived more than double the orbital time and triple the radiation exposure originally intended.

Cassini-Huygens was launched by a Titan IV-Centaur rocket from Cape Canaveral on 15 October 1997. It completed two swingbys of Venus (26 April 1998 and 24 June 1999) and one of Earth (18 August 1999) on its way to Jupiter. It is due to arrive at Saturn on 1 July 2004, and the European Huygens probe is scheduled to enter Titan's atmosphere on 27 November 2004.

Jupiter is a giant gas planet, which is composed mainly of hydrogen and helium. Some 1,400 Earths would fit inside it. Its magnetosphere is so large that it stretches all the way to Saturn. There are 17 known moons, the largest of which are Ganymede, Callisto, Europa and Io.

UK investment in the Cassini-Huygens mission totals 7.4 million pounds, including 4.48 million pounds for the orbiter experiments and 2.92 million pounds for the Huygens probe experiments. Most of this is provided by the Particle Physics and Astronomy Research Council (PPARC).